Tubular solid oxide fuel cells (SOFCs) are the most extensively demonstrated of the many designs proposed for SOFCs. In these structures, a multi-layer tube is fabricated with cathode, electrolyte, and anode layers. Tubes that are supported by anodes, cathodes, and electrolytes each have been proposed in the literature and demonstrated. Electrolyte- and cathode-supported tubes, in both circular and flat tube configurations, have been demonstrated, as have anode-supported tubes.
In tubular SOFCs, fuel or air is flowed down the center of the tube, depending on whether the tube is anode- or cathode-supported, while the complementary gas mix is flowed outside the tube. Such tubes can have open or closed ends and are typically sealed outside the reaction zone of the SOFC. Conventional tubular cells typically suffer from low volumetric or gravimetric power density because large tubes do not pack well and have a low surface area to volume ratio.
Microtubular SOFCs, in which small-diameter (i.e., <5 mm) tubes of electrolyte are slurry coated with cathode and anode components, overcome some of the disadvantages of conventional tubes. Sealing of small diameter microtubes is simpler than sealing of conventional tubes. Microtubular cells also overcome the low surface area to volume ratio associated with conventional tubular cells. However, microtubular cells require complex manifolding and electrical interconnection schemes, which makes scaling to large power stacks difficult.
Planar SOFCs, which may be supported by either the electrode or the electrolyte, also have been demonstrated extensively. Electrode-supported cells have a thick electrode component that provides the mechanical load-bearing member of the cell and a thin electrolyte layer that dramatically reduces electrolyte ohmic resistance in the cell and allows operation at intermediate temperatures (e.g., T<800° C.). Electrode-supported SOFCs typically are produced by co-sintering the support electrode material and a thin coating of electrolyte material. The electrode support is typically tape cast, calendared or slip cast, although other preparation methods have been demonstrated. The thin electrolyte can be deposited in a number of ways, including but not limited to lamination of electrolyte tape, screen printing, calendaring, and spray deposition. Electrode-supported cells preferably have a sintered electrolyte layer that is less than twenty microns in thickness after sintering with the electrolyte layer being well-adhered to the electrode support.
Electrode-supported planar SOFCs include both cathode- and anode-supported cells. Cathode-supported cells have the potential to be lightweight and lower in cost than anode-supported cells. However, processing of cathode-supported cells is difficult because the co-firing of most cathode materials in contact with an electrolyte produces insulating intermediate compounds. Anode-supported electrolytes are perhaps the most widely evaluated cell geometry for low temperature operation. Processing of anode-supported cells is comparatively easy because sintering temperatures in excess of 1300° C. can be used to achieve dense electrolytes without concern for interaction between the anode material and the supported electrolyte.
Planar anode-supported cells are particularly attractive for mass market, cost driven applications because of their high areal power density. Performance of anode-supported cells at 700° C. has been demonstrated to be over 1 W/cm2 in small cells at low fuel utilization. With appropriate seal and interconnect technology, power densities greater than 0.4 W/cm2 have been reported for anode-supported cell stacks. The planar structure also offers the advantage of packing efficiency. However, anode-supported cells are not without drawbacks. When conventional nickel oxide/yttrium stabilized zirconia (NiO/YSZ) composites are used as support materials, the reduction of NiO to nickel metal creates stress in the electrolyte layer, which may cause considerable deformation during this reduction process. Operating planar anode-supported cells at high power density and high fuel utilization also is difficult; the thick porous layer prevents rapid diffusion of steam away from the electrolyte and results in increased cell ASR at high current density.
Alternatively, electrolyte-supported planar cells have an electrolyte layer that provides the mechanical strength of the cell. The electrolyte layer can be produced by tape casting or other methods. Electrodes are typically applied to the electrolyte later by screen printing or spray coating and fired in a second step. To achieve strong electrode adhesion, the ink particle size, composition, and surface area must be tailored to the target firing temperature and controlled during fabrication. Electrodes can be sintered in two separate stages or simultaneously, depending upon the requisite temperatures for the cathode and anode. In many cases, the anode ink is fired first because it is more refractory and more difficult to sinter, and the cathode ink applied and fired in a second step at a lower temperature to minimize the possibility of electrolyte/cathode interaction. Electrolyte-supported cells offer numerous advantages in the production of SOFCs. The sealing of electrolyte-supported cells is expected to be simpler than for electrode-supported planar cells because a dense electrolyte perimeter can be preserved during electrode printing, which provides a dense, smooth surface for sealing operations. Electrolyte-supported cells also have good stability during reduction. Because only a thin layer of anode is affected by the reduction process, this process generally has little impact on cell mechanical stability. Gas diffusion in and out of the thinner anode layer also makes steam diffusion limitation less of a concern.
However, electrolyte-supported cells often exhibit much higher area specific resistance values than electrode-supported cells because the electrolyte typically exhibits lower bulk conductivity than the anode or cathode materials. To compensate for this higher area-specific resistance, the operating temperature for electrolyte-supported cells generally is higher than anode-supported cells using the same materials set. The higher operating temperature of the electrolyte-supported cells can be a drawback, particularly for developers wishing to use metallic interconnect materials.
In spite of more than thirty years of continuous research in the area of SOFCs, these systems remain far from commercialization. Until improved SOFC cell designs are identified that address the shortcomings of existing cell structures, it will be difficult for SOFCS to overcome the commercialization barriers presented by conventional energy production routes. Considering planar cells in particular, a cell that delivers high performance, high mechanical strength, and easier sealing than current electrolyte- or anode-supported cells is essential in providing an avenue for commercialization of SOFCs.